Novel concept of interferometric control of the moving coil in watt balance

Abstract

One of the most promising projects for the new definition of the kilogram is the watt balance. It uses a two-mode procedure: static mode and dynamic mode. One of the key points is to control the velocity, the yaw and the pitch of a moving coil during the dynamic mode. We have developed a specific method based on the use of a homemade double pass heterodyne interferometer, a two-level translation stage, and a home-made high frequency phase shifting electronic circuit. This system will replace the commercial heterodyne system (laser head and interferometer) used at present time which is limited in power and has polarization limitations. In this paper, we propose a novel and specific heterodyne interferometer able to ensure all previous demands to control the displacements of the moving coil in order to reduce the velocity noise. The laser source is powerful enough to ensure multiple measurements axis. The laser is frequency stabilized on iodine atomic transition around λ=532 nm to ensure traceability. All the polarization defaults of the classical heterodyne interferometer have been minimized in order to reduce non-linearity effects. The most original point is that the reference beam is locked on an external electronic reference, which allows to be less sensitive to vibration and polarization defects of optical components from the laser head to the vacuum enclosure where the interferometer is placed.